Business Segments - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Interest income, including loan fees									$ 252,903	$ 232,773	$ 262,222
Credit for funds provided									0	0	0
Total interest income									252,903	232,773	262,222
Interest expense									16,389	16,507	25,272
Charge for funds used									0	0	0
Total interest expense									16,389	16,507	25,272
Net interest income	61,175	61,238	57,159	56,942	55,109	53,973	52,595	54,589	236,514	216,266	236,950
Provision for loan losses	0	(1,000)	(7,600)	(7,500)	(6,800)	(3,750)	(6,200)	0	(16,100)	(16,750)	0
Net interest income after provision for loan losses									252,614	233,016	236,950
Noninterest income									36,412	25,287	15,903
Noninterest expense									126,229	114,028	138,160
Debt termination expense									0	0	20,379
Earnings before income taxes									162,797	144,275	114,693
Segment assets	7,377,920				6,664,967				7,377,920	6,664,967	6,363,364
Operating segments [Member] | Centers [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees									142,467	139,369	147,726
Credit for funds provided									31,188	26,754	25,764
Total interest income									173,655	166,123	173,490
Interest expense									6,494	6,138	7,162
Charge for funds used									5,123	3,193	3,204
Total interest expense									11,617	9,331	10,366
Net interest income									162,038	156,793	163,124
Provision for loan losses									0	0	0
Net interest income after provision for loan losses									162,038	156,793	163,124
Noninterest income									20,513	20,733	22,807
Noninterest expense									47,871	45,268	44,780
Debt termination expense									0	0	0
Earnings before income taxes									134,680	132,258	141,151
Segment assets	6,006,415				5,405,939				6,006,415	5,405,939	5,196,244
Operating segments [Member] | Treasury [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees									71,369	53,234	56,559
Credit for funds provided									0	0	0
Total interest income									71,369	53,234	56,559
Interest expense									9,590	9,700	15,396
Charge for funds used									54,885	45,269	41,270
Total interest expense									64,475	54,969	56,666
Net interest income									6,894	(1,735)	(107)
Provision for loan losses									0	0	0
Net interest income after provision for loan losses									6,894	(1,735)	(107)
Noninterest income									0	2,094	0
Noninterest expense									784	714	729
Debt termination expense									0	0	20,379
Earnings before income taxes									6,110	(355)	(21,215)
Segment assets	3,268,551				2,866,760				3,268,551	2,866,760	2,642,445
Operating segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees									39,067	40,170	57,937
Credit for funds provided									46,770	41,987	31,505
Total interest income									85,837	82,157	89,442
Interest expense									305	670	2,714
Charge for funds used									17,950	20,279	12,795
Total interest expense									18,255	20,949	15,509
Net interest income									67,582	61,208	73,933
Provision for loan losses									(16,100)	(16,750)	0
Net interest income after provision for loan losses									83,682	77,958	73,933
Noninterest income									15,899	2,460	(6,904)
Noninterest expense									77,574	68,046	72,272
Debt termination expense									0	0	0
Earnings before income taxes									22,007	12,372	(5,243)
Segment assets	753,068				710,844				753,068	710,844	637,976
Eliminations [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees									0	0	0
Credit for funds provided									(77,958)	(68,741)	(57,269)
Total interest income									(77,958)	(68,741)	(57,269)
Interest expense									0	0	0
Charge for funds used									(77,958)	(68,741)	(57,269)
Total interest expense									(77,958)	(68,741)	(57,269)
Net interest income									0	0	0
Provision for loan losses									0	0	0
Net interest income after provision for loan losses									0	0	0
Noninterest income									0	0	0
Noninterest expense									0	0	0
Debt termination expense									0	0	0
Earnings before income taxes									0	0	0
Segment assets	$ (2,650,114)				$ (2,318,576)				$ (2,650,114)	$ (2,318,576)	$ (2,113,301)